Wind vane steering control of sailing craft is well known and has been used for many years particularly on sailing yachts of the historically conventional type, that is, of the single hull or mono-hull type. It appears that wind vane steering was first employed to any great extent on sailing models. In more recent years, such steering control has been adapted to full size sailing craft, mostly yachts in the range of size between twenty and one hundred feet in overall length, particularly those sailed "short-handed." There are many different types of wind vane steering devices, but they all have the common function of steering the craft on a pre-selected compass course by responding to the direction of the apparent wind encountered by the craft on such course and transmitting such response to an underwater steering rudder, either the craft's own rudder or an auxiliary rudder provided for the purpose.
The apparent wind is the wind which is sensed by a sail or power craft as it moves through the water. Its velocity and direction are the resultants of the forces created by the velocity and direction of the true wind and by the direction and speed of movement of the craft through the water. For instance, a speedboat moving at sixty knots in a five knot true wind from one side senses an apparent wind from almost dead ahead. Fast sailing ice boats encounter similar apparent winds. Under ideal conditions the direction and velocity of the true wind will remain steady and the direction and speed of the craft's hull through the water will also remain steady so that the craft will encounter an apparent wind which is also steady in direction and velocity.
However conditions at sea are never constant. A sailing hull is subjected to variations in velocity of the true wind which in turn create variations in relative direction and velocity of the apparent wind. Generally speaking, an increase in the velocity of the true wind will cause the direction from which the apparent wind is coming to draw aft because the forward component due to the craft's movement is comparatively reduced. That is, the angle of the direction from which the apparent wind is coming relative to the ship's heading will increase. Conversely, a drop in the velocity of the true wind will cause the apparent wind to draw ahead or forward thus decreasing the angle of its direction relative to the craft's heading.
A change in the speed of the craft through the water also changes the angle of the apparent wind in that an increase in the speed of the hull through the water will tend to make the apparent wind draw ahead and a decrease in speed will cause it to draw aft or back. However a conventional sailing craft rarely exceeds a speed of ten knots and most sailing yachts usually sail below a speed of seven or eight knots in average conditions so that they encounter very few changes in apparent wind direction due to changes in hull velocity. Consequently the changes in apparent wind direction they encounter are caused almost entirely by changes in true wind velocity or direction or because the craft has been thrown off its course due to wave action.
Vane type steering devices work satisfactorily on conventional mono-hull sailing craft because such vessels are generally slow to respond to minute rudder changes caused by momentary changes in wind velocity or wave action. Thus, relatively speaking, the somewhat inert turning reaction of the hull creates a damping action which tends to resist what would otherwise be rapid and erratic course changes due to variations in the velocity of the wind and wave action.
It will be understood that with a relatively heavy mono-hull craft, acceleration and deceleration are relatively low and do not produce rapid changes in apparent wind direction. It will also be understood that a change in true wind direction unless relatively temporary will require re-setting the wind vane.
Recent experience with high speed multi-hull sailing craft has shown that the use of a wind vane alone for steering control is highly unsatisfactory under some conditions.
Such multi-hull craft are capable of achieving relatively very high speeds which are of the order of ten to thirty knots as opposed to the below ten knot speeds of conventional mono-hull vessels. Multi-hull craft are very light in weight compared to conventional craft. When they encounter a puff of wind, that is, a sudden increase in true wind velocity, they are capable of high acceleration, say from five to eighteen knots in a very short time and, conversely, of high deceleration when the wind drops, creating a variation in apparent wind angle due to changes of hull speed through the water not ordinarily encountered by conventional craft.
Such light weight craft are much more sensitive to small rudder changes due to their inherent light weight and because there is very little of any of the hulls of the craft below the surface of the water to resist turning influences such as are encountered with conventional sailing craft. The result is that, particularly when sailing at high speeds in strong winds, wind vane steering control alone produces very rapid and erratic course changes. This is because inherently there is very little damping effect exerted by a high speed sailing hull as compared to that exerted by a slower turning less responsive mono-hull acting to reduce erratic course changes. Otherwise stated, a relatively slow moving sailing hull requires much more rudder angle and a longer time to cause it to turn than a high speed, light weight sailing hull which requires very little rudder angle to cause radical course changes.
Something additional is required in conjunction with a wind vane steering control to dampen and stabilize high speed sailing craft against such erratic course changes as result when a wind vane is used alone.
The apparatus of my invention provides such a stabilizing and damping effect and "irons out" rapid momentary undesired course changes with the result that a wind vane steering control in combination with my apparatus will operate satisfactorily as a steering control when applied in high speed sailing craft.